Peracetic acid (denoted PAA or HOOAc herein) has long been recognized for its utility in a wide variety of end uses. In water and wastewater disinfection, for example, PAA destroys microorganisms and pathogens harmful to the public and the environment without producing toxic by-products or leaving chemical residuals. In bleaching applications, PAA yields higher levels of brightness without degrading fiber strength. Other applications, including equipment sanitizing, grain and soil sterilization, and chemical synthesis profit from the benefits of PAA over the alternatives.
PAA is currently produced commercially as an equilibrium mixture of hydrogen peroxide, acetic acid, water, and sulfuric acid with trace amounts of stabilizers (denoted equilibrium peracetic acid, or eq-PAA herein). The active peracetic acid content is typically controlled from 5 to 35%, by weight, depending on the particular end use.
However, there are several shortcomings with eq-PAA that limit its practical utility. For example, the use of eq-PAA is limited by its inherent instability and safety considerations, particularly at high concentrations. Thirty-five percent (35%) solutions are flammable under the National Fire Prevention Association (NFPA) standards. Commercial transport regulations restrict the concentration to less than 35% when shipped over public avenues, however, manufacturers typically limit the concentration to 15% except for special circumstances.
As an equilibrium product, Eq-PAA contains substantial quantities of unreacted raw materials. These unreacted materials represent economic waste that can increase the cost for the active PAA by a factor of two to three.
There are many applications where the presence of these unreacted materials discourages the use of eq-PAA. For example, in conventional wastewater treatment plants the unreacted hydrogen peroxide (H2O2) and acetic acid (HOAc) will contribute significantly to the loading properties in final discharges, including toxicity, Chemical Oxidation Demand (COD), Biochemical Oxygen Demand (BOD), and Total Organic Carbon (TOC), etc., all of which must meet local and federal discharge permits (for example NPDES, National Pollutant Discharge Elimination System) promulgated under the United States Of America Clean Water Act.
Several methods have been proposed to moderate the limitations of eq-PAA. In U.S. Pat. No. 5,122,538 to Lokkesmore et al., a method is disclosed to produce equilibrium peracetic acid products on-site at the point of use. The method utilizes a non-swelling acid exchange resin as a catalyst to produce peracetic acid from acetic acid and hydrogen peroxide. A drawback to this method is the peracetic acid product contains substantial amounts of acetic acid and hydrogen peroxide. The prolonged equilibration time (several hours) necessitates large inventories of peracetic acid that present additional storage hazards requiring special precautions. This method for on-site production and storage retains the adverse impacts on cost, safety and the environment.
A more complete solution involves separating and recycling the non-PAA components in eq-PAA. Among these is distilled PAA (denoted aq-PAA herein) which has been available since the 1950's.
In an early U.S. Pat. No. 3,264,346 to Weiberg, et al., a process is described for producing an aqueous solution of peracetic acid by distilling off the solution from a reaction medium containing acetic acid, hydrogen peroxide, sulfuric acid, water and peracetic acid. The use of this process calls for a molar ratio of hydrogen peroxide to acetic acid of from 5:1 to 15:1.
In Swern, D., “Organic Peroxides”, Vol. 1, John Wiley & Sons, New York, 1970, pp. 349–351, a process is described for the production of peracetic acid by distilling off peracetic acid from an aqueous reaction medium containing acetic acid, hydrogen peroxide, peracetic acid, and sulfuric acid catalyst in substantial equilibrium.
Other art describes specific aspects of the aq-PAA production process such as distillation, purge disposal, and product stabilization.
One example of the distillation aspect is U.S. Pat. No. 5,886,217 to Pudas, which describes the production of eq-PAA and distilling off aq-PAA continuously based on the amount of thermal energy applied to the reaction medium. The purpose is to produce the maximum yield of aq-PAA from a commercial production viewpoint by increasing or maintaining a high level of thermal energy input.
In EP 0789016, a method is disclosed for producing peracetic acid by reacting hydrogen peroxide and acetic acid in an aqueous medium that is continuously supplied with more than 0.2 KW/kg of thermal energy while the peracetic acid produced is continuously separated and removed by distillation. The reaction medium circulates in the heating device with the aid of a pump, which increases the pressure of the reaction medium to allow temperature rise with boiling in the circulation loop.
Pudas U.S. Pat. No. 5,886,217 also describes a heat exchanger circulating device to supply the thermal energy to the reaction medium. A drawback to this method is that the heat exchanger circulating device circulates the reaction medium up to 200 times per hour and basically serves as a continuously mixed reactor and heater.
In EP 1004576 A1, a method is disclosed for producing peracetic acid by reacting hydrogen peroxide and acetic acid in an aqueous medium in the presence of an acid catalyst and continuously distilling off the peracetic acid, the molar ratio of hydrogen peroxide to acetic acid in the reaction medium ranging from 0.6:1 to 4:1, respectively, and the reaction medium being circulated through a thermosyphon reboiler by natural convection boiling.
Another derivation involves reaction medium sparging, wherein fresh acid catalyst is fed continuously into the reaction medium while withdrawing a similar volume continuously to purify the medium of impurities threatening the safety of the process. One example of medium sparging is described in U.S. patent application No. 20020177732 to Pohjanvesi, et al., which describes improvements by feeding the catalyst continuously into the reaction medium by withdrawing a portion of the medium as a bottom product. The resulting medium is distilled into aq-PAA at maximum yields.
EP 1247802 describes a method for disposing of the acid purge stream by neutralizing the remaining sulfuric acid and acetic acid therein and combining the stream with the distilled peracetic acid product. A slightly better utilization of the raw materials is achieved in addition to eliminating a waste stream.
Another example is EP 98203946.3, which additionally describes the use of stabilizers and chillers to stabilize the aq-PAA.
Another example is described in U.S. patent application No. 20020193626 to Pohjanvesi, et al., which describes using a base to neutralize the unreacted acids whereby the distilled PAA product is stabilized. Significantly, all these acid neutralization processes suffer from common drawbacks, including the addition of a stoichiometric equivalent amount of base and the introduction of conjugate salts that are not desirable in most applications.
The inherent instability of PAA leads to another shortcoming of aq-PAA: its transportability. When stored at ambient temperatures (for example 20° C.), aq-PAA quickly reverts to H2O2 and HOAc (approximately 1.5% decomposition per day). When stored at low temperatures (for example 0° C.), the reverse reaction is considerably slower and the product may retain a reasonable shelf life (approximately 0.3% decomposition per day). Thus, aq-PAA is not sufficiently stable for prolonged periods without elaborate refrigeration equipment and controls, which greatly complicates its distribution and storage.
To date, the United States Department of Transportation (USDOT) has not permitted the transportation of eq-PAA in containers greater than 300 gallon intermediate bulk containers (IBC's), while regulations for the transport of aq-PAA have not been delineated. Therefore, PAA cannot be shipped in bulk quantities and at prices competitive with conventional bulk chemicals such as sodium hypochlorite for disinfection.
In summary, the prior art related to aq-PAA production is concerned with producing both the highest yields of PAA and addressing the stability of PAA during production, handling, and storage regardless of the end use requirements.
Further, to date aq-PAA production technology has been applied commercially only within specialized chemical manufacturing facilities. These processes are designed for large scale production of PAA using large quantities of reacting solutions and large head spaces above the solution at the base of the distillation column. The principal hazard associated with the technology is the potential for unstable conditions (possible vapor phase explosions) to exist within the vapor space above the surface of the liquid in the reactor.
Thus, none of the described prior art teaches of a small to medium scale process suitable for practical on-site production of aq-PAA (i.e., one dedicated to specific end-use applications). There remains a need for a safer and more cost effective method for producing and applying PAA into various applications without releasing significant waste products. It would be especially desirable to develop a continuous process capable of producing aq-PAA with variable on-demand controls to suit a wide variety of applications, especially a process that can operate safely and efficiently under a range of processing conditions and with minimal human intervention.